1. Field of the Invention
The present invention relates to electro-optical systems for reading a one or two-dimensional bar code symbology, and more particularly, to a generic handheld symbology scanner that can be adapted for various uses by selection of a removable modular optical unit.
2. Description of Related Art
Within the automatic data identification and collection industry, electro-optical systems are commonly used to decipher data symbols printed on objects in order to identify information regarding the objects. A conventional bar code symbol represents a one-dimensional form of symbology, and comprises a pattern of vertical bars of various widths separated by spaces of various widths. Since the bar and space elements have different light reflecting characteristics, a reader can convert the symbology into an electrical signal by analyzing the light reflected from the symbol. The electrical signal can then be decoded to provide an alphanumeric representation of the symbol which identifies the object. Bar code symbols of this nature are now in common usage in various types of applications, such as inventory management and control, point of purchase identification, logistical tracking systems, or time and attendance record keeping.
A bar code scanner typically uses a light source that is drawn, or scanned, across the bar code field. Since the bar code symbol is often disposed on the object to be identified, it is desirable for the scanner to be included in a handheld or portable device so that the scanner can be brought to the object. Light emitting diodes (LEDs) are often utilized to provide the light source due to their light weight and low power requirements. The operator can physically move the LED across the bar code field, such as by use of a light pen. Though advantageous for some applications, these LED scanners have a rather limited scanning range. Greater scanning range can be achieved by a bar code scanner that includes movable mirrors to automatically articulate a light beam from a laser emitting source back and forth at a high rate to scan the light beam across the bar code field. Another scanning approach is to utilize a one-dimensional charge-coupled device (CCD) having a single one-dimensional row of imaging elements. The CCD device converts the printed information of the bar code symbol into an electrical signal representation. A one-dimensional CCD scanner can read an entire bar code symbol at once without requiring movement of the light source, as is necessary with the LED or laser emitting systems described above. As with the articulated laser scanners, however, one-dimensional CCD scanners are orientation dependent and must be aligned with the bar code symbol to accurately collect the information.
Since the conventional one-dimensional symbology requires a relatively large amount of space to convey a correspondingly small amount of data, so-called two-dimensional symbologies have been developed. A two-dimensional symbology may comprise a matrix that occupies a uniform amount of space having a generally rectangular or square shape. Instead of bars and spaces, round or square dots disposed at particular rows and columns of the matrix correspond to the characters being conveyed. As a result, a matrix symbology can compress significantly more data into a given volume of space than a conventional one-dimensional bar code. Examples of commercially available two-dimensional symbologies include Code One, Data Matrix, and PDF417.
Though some two-dimensional symbologies may be read using the conventional scanners described above, another approach is to convert the two-dimensional symbol into pixel information that is deciphered into the alphanumeric information represented by the symbology data. Such two-dimensional scanners may utilize two-dimensional CCD devices to obtain an optical image of the symbol and convert it into an electrical signal. These two-dimensional CCD scanners are not orientation dependent like the one-dimensional CCD or laser scanners, since the electrical signal may be processed to determine the rotational orientation of the symbol, remove any extraneous information, and thereby recover the alphanumeric information of the symbol. Thus, these two-dimensional scanners provide greater flexibility to the operator by permitting a symbol to be effectively read from a wide assortment of angles and orientations. An additional advantage of these two-dimensional scanners is that they can also be utilized to read one-dimensional symbology data, such as a conventional bar code symbol.
A drawback of each of the types of one and two-dimensional scanners described above is that they are not interchangeable and thus cannot be converted from one type to another. Each scanner type is optimized to use only one of the aforementioned optical sensors (e.g., LED, laser, CCD, etc.), which is mechanically and electrically integrated into the scanner in a permanent and non-removable manner. Since each scanner type has certain symbol reading applications for which it is most proficient and best suited, a user would select a scanner that is optimized for each particular application. Even if a user only utilizes scanners of the two-dimensional CCD type, there are differences in optical characteristics between individual scanners. For example, one type of CCD scanner may include focusing systems (i.e., lenses) optimized for scanning distances of less than one foot, while another type of CCD scanner may be optimized for distances of three to five feet. While a user can be ready for any scanning application by maintaining a supply of various types of scanners, it can be appreciated that this greatly increases the cost and complexity of operating an automatic data identification and collection system.
Accordingly, a critical need exists for an interchangeable scanner that can be optimized for various different applications. Moreover, such an interchangeable scanner should be able to operate with any type of optical sensor, and have a wide assortment of focusing characteristics.